The ScriptTagHelper was incorrectly suppressing <script type="importmap"> tags that contained explicit user-provided content, causing them to render as nothing in ASP.NET Core 9.0.

Problem

When users wrote importmap scripts with explicit content like:

<script type="importmap">
{
  "imports": {
    "jquery": "https://code.jquery.com/jquery.js"
  }
}
</script>

The ScriptTagHelper would completely suppress the output instead of preserving the user's content. This regression broke existing applications that relied on explicit importmap definitions.

Root Cause

The tag helper was being overly aggressive - it processed all script tags with type="importmap" and when no ImportMapDefinition was found in endpoint metadata, it would call output.SuppressOutput() regardless of whether the user had provided explicit content.

Solution

Modified the ScriptTagHelper to check for existing content first using await output.GetChildContentAsync():

• If existing content is found (not empty/whitespace) → preserve it and don't process further
• If no existing content → apply normal importmap logic (generate from definition or suppress if none found)

This approach ensures that user-provided content is never replaced, while maintaining all existing functionality for automatic importmap generation.

Implementation Details

• Added ProcessAsync method that properly handles the async content checking
• Maintained backward compatibility by keeping the original Process method
• Updated public API to reflect the new async method

Testing

Added comprehensive tests to verify:

• Explicit importmap content is preserved when users provide it
• Output is still suppressed when no content and no definition exists
• All existing functionality remains intact

Fixes #58973.

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